1. Technical Field
Various embodiments generally relate to a memory device, a refresh method, and a system including the same. More particularly, various embodiments may relate to a technology capable of reducing a refresh current of a memory device.
2. Related Art
Recently, there are demands for a DRAM with a large capacity in mobile electronic products including smart phones and the like. In general, in a semiconductor memory device such as a DRAM, data stored in a memory cell may be changed by a leakage current. Accordingly, in order to periodically recharge the data stored in the memory cell, a refresh operation is required.
That is, a memory cell of a dynamic semiconductor memory such as a DRAM stores data on a capacitive element. Due to charge leakage from the capacitive element, the memory cell should be periodically refreshed. A refresh process typically includes a step of performing a read operation in order to read a level of charge stored in the memory cell as is.
Particularly, a semiconductor memory device such as a DDR SDRAM (Double Data Rate Synchronous DRAM) includes a plurality of memory banks for storing data, wherein each of the plurality of memory banks includes tens of millions or more of memory cells. Each memory cell includes a cell capacitor and a cell transistor, and the semiconductor memory device stores data through an operation for charging or discharging charge in/from the cell capacitor.
The amount of the charge stored in the cell capacitor should always be constant, ideally, if there is no separate control. However, actually, due to a voltage difference with a peripheral circuit, the amount of the charge stored in the cell capacitor varies.
That is, charge may be discharged in the state in which the cell capacitor has been charged, or charge may be introduced in the state in which the cell capacitor has been discharged. As described above, a charge in the charge amount of the cell capacitor represents a charge in data stored in the cell capacitor, which represents loss of the stored data. In order to substantially prevent such data loss, the semiconductor memory device performs a refresh operation.
With the passage of time, different types of refresh methods have been developed. In a normal auto-refresh method, a refresh timer exists outside a memory chip and the memory chip performs a refresh operation in response to a periodical refresh command supplied by a controller.
In a self-refresh method, a refresh timer exists inside a memory chip and all memory chips require a refresh start command from a controller.